Interpretive Summary: Wild annual sunflower is native to North America where it is widely distributed in the US. This species has become naturalized in central Argentina and is scattered over an area of about 12 million acres. Since its establishment 60 years ago it has continuously increased in area, behaving as an invasive species providing evidence it will continue to spread. This newly domesticated species can be a valuable germplasm resource for cultivated sunflower. No studies have been done to compare the U.S. and naturalized Argentine populations. Nine populations representative of different geographic regions of central Argentina were compared with 17 populations from the U.S. (center of origin) in a common garden study at Bahía Blanca, Argentina using 47 phenotypic traits. The nine invasive wild populations were differentiated among themselves and from the native populations by plant and life cycle traits, oil composition, inflorescence, and achene morphology. Populations from both continents shared traits related to domesticated sunflower, such as bract width over 0.8 cm, but the frequency of this trait was higher in populations from Argentina. The genetic diversity found in the invasive populations from Argentina reflected about three-fourths of the diversity of those from the center of origin, even though the environmental conditions and habitats of the Argentine populations was half of those available in the North American populations. The current findings demonstrated that the invasive wild sunflower populations in Argentina have developed a high degree of variability and genetic diversity which could be a source of useful novel genetic resources for sunflower crop improvement.

Technical Abstract:
The sunflower's wild relative, Helianthus annuus L., is a non-native invader in several regions of the world. It was introduced as experimental forage in central Argentina six decades ago where it probably escaped and developed extended populations coexisting with the sunflower crop. If the invasive taxon was diffused without modifications, it would be expected to have phenotypic similarities with its parental sources. Nine populations representative of different geographic regions of central Argentina were compared with 17 populations from the USA (center of origin) in a common garden study at Bahía Blanca, Argentina using 47 phenotypic traits. The nine invasive wild populations were differentiated among themselves and from the native populations by plant and life cycle traits, oil composition, inflorescence, and achene morphology. Populations from both continents shared traits related to domesticated sunflower, such as bract width over 0.8 cm, but the frequency of this trait was higher in populations from Argentina. The high variability in the wild H. annuus populations from Argentina did not reveal any founder effects. The biodiversity found in the invasive populations reflected about three-fourths of the phenotypic variability of those from the center of origin, even though the environmental conditions of the Argentine habitats represented only half of the variability present in the North American habitats. The current findings demonstrated that the invasive wild sunflower populations have developed a high degree of variability, which could be a source of a novel biodiversity useful as a genetic resource for sunflower crop improvement.